Gravitational collapse is the collapse of a very massive body because of its attraction for itself.  According to Newton's law of Universal gravitation, as the distance decreases, the force of gravity increases.  If the gravitational force of attraction becomes greater than the electromagnetic force of repulsion, the whole mass would continue to contract until the original volume of mass becomes a fantastically massive point called a singularity.  This singularity is surrounded by an invisible spherical boundary known as the event horizon.   Any matter or radiation within the event horizon cannot escape the influence of the singularity.   Thus the event horizon is a one way boundary, since matter and radiation can enter but cannot leave. Schwarzschild Radius is the radial distance the event horizon is located from the singularity.  It equals twice the gravitational constant times the mass of the collapsed star divided by the square of the speed of light.  The space inward from the event horizon is a black hole.  Schwarzschild radius :
Schwarzschild radius is the radius below which the gravitational attraction between the particles of a body must cause it to undergo irreversible gravitational collapse. This phenomenon is thought to be the final fate of the more massive stars.
The gravitational radius (R) of an object of mass M is given by the following formula, in which G is the universal gravitational constant and c is the speed of light: R = 2GM/c2 . For a mass as small as a human being, the gravitational radius is of the order of 10-23 cm, much smaller than the nucleus of an atom; for a typical star such as the Sun, it is about 3 km (2 miles).
 

For more information click here:     Information about Black Holes

For more information about Black Holes and Neutron Stars, click here:    Black Holes and Neutron Stars
 

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